1. Field of the Invention
The present invention relates to a two-wavelength antireflection film, and more particularly to a two-wavelength antireflection film capable of effective antireflection in two wavelength regions, for example, in an ultraviolet wavelength region of 200-300 nm and a visible wavelength region of 600-700 nm.
2. Related Background Art
In recent semiconductor device production, for achieving a higher resolution, there have been proposed various projection exposure apparatus (steppers) utilizing the exposure light of a wavelength shorter than that of g- or i-line, for example, the ultraviolet light of a wavelength of 248 nm emitted from an excimer laser.
Alternatively, in order to achieve high precise relative alignment of the reticle and the wafer, there have been proposed various alignment systems employing light where the wavelength is different from that of the exposing light and which is within the visible wavelength region in order to enable observation of the wafer surface but is non-actinic to the photoresist, for example, the light of a wavelength of 632.8 nm emitted from a He-Ne laser.
In such projection exposure apparatus, the faces of the mirrors and lenses constituting the optical systems therein are provided with thin films of predetermined transmittances (or reflectances) in the ultraviolet and visible wavelength regions. For example the Japanese Patent Laid-open Application Nos. 63-113501, 2-127601 and 2-12605 propose antireflection films capable of certain antireflection in the ultraviolet and visible wavelength regions.
The conventional antireflection film is composed, in a part thereof, of a highly refractive material of a refractive index of 1.9 or higher, such as Y.sub.2 O.sub.3.
A highly refractive material, such as Y.sub.2 O.sub.3, generally has a high absorbance in the ultraviolet region below 300 nm. For this reason, when a KrF excimer laser (wavelength 248 nm) is employed as the light source for achieving a high resolution in the projection exposure apparatus (stepper) for the semiconductor device production, the transmittance of the projection lens is lowered, and the light absorption raises the temperature of the optical elements, thereby causing a variation in the optical performance thereof.
Besides the light absorption causes damage to the antireflection film, or a change in the film configuration, whereby the desired spectral characteristics (antireflectivity) can no longer be obtained.